1. Field of the Invention
The present invention relates to the monitoring of an evaporative purge system of a motor vehicle having a fuel tank connected to an internal combustion engine for the purpose of determining whether the purging system is emitting hydrocarbon emissions into the atmosphere.
2. Description of Related Art
Evaporative emission control systems are widely used in internal combustion engine powered motor vehicles to reduce evaporated fuel; i.e. fuel vapor emissions, from the fuel tank to the atmosphere which, allegedly, contributes to smog. Evaporative purge systems typically include a vapor management valve to control fuel vapor flow to the engine from a vapor storage canister, vapor flow control valves on the fuel tank, as well as hoses or conduits connecting the components. In the event that one or more of the purge system valve components degrades, an emission of fuel vapors may occur, resulting in reduced engine performance and possible release of vapors into the atmosphere. U.S. Pat. No. 5,614,665 describes various modes that can result in emission of fuel vapor to the atmosphere from the evaporative purge system.
Various techniques are known for monitoring the evaporative purge system to determine its functioning. Monitoring techniques have been used to monitor the evaporative system in either the cruise operating mode or the idle operating mode of the motor vehicle. Aforementioned U.S. Pat. No. 5,614,665 discloses a monitoring method and system that involve sealing the evaporative purge system to build up pressure due to evaporation of fuel vapor and monitoring for a pressure change above a predetermined threshold.
As the monitored system emission size is decreased to further reduce vapor emissions, longer monitoring test times are required. Long test times in a vehicle subject to altitude changes with a sealed evaporative purge system can produce false system fault indications.